1. Technical Field
The present invention relates to a liquid developer and an image forming apparatus, and in particular relates to a liquid developer and an image forming apparatus that can use the liquid developer.
2. Related Art
As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known two types. One type of such a developer is known as a dry toner which is formed of a material containing a coloring agent such as a pigment or the like and a binder resin, and such a dry toner is used in a dry condition thereof.
The other type of such a developer is known as a liquid developer (liquid toner) which is obtained by dispersing toner particles into a carrier liquid having electric insulation properties (one example of such a liquid developer is disclosed in JP-A 7-152256).
In the developing method using such a dry toner, since a solid state toner is used, there is an advantage in handleability thereof. On the other hand, however, this method involves problems in that an adverse effect against a human body is likely to be caused by toner powder, contamination is likely to be caused by dispersal of toner powder, and toner particles are likely to be massed together in a cartridge.
Further, in such a dry toner, since aggregation of toner particles is likely to occur in the producing process thereof, it is difficult to obtain toner particles each having a sufficiently small diameter. This means that it is difficult to form a toner image having high resolution.
Furthermore, there is also a problem in that when the size of the toner particle is made to be relatively small, the problems resulted from the powder form of the dry toner described above become more serious.
On the other hand, in the developing method using the liquid developer, since aggregation of toner particles in the liquid developer is effectively prevented, it is possible to use very fine toner particles and it is also possible to use a binder resin having a lower softening point (a low softening temperature) than a softening point of resin material used for a dry toner.
As a result, the method using the liquid developer has such advantages as good reproducibility of an image composed of thin lines, good tone reproducibility as well as good reproducibility of colors. Further, the method using the liquid developer is also superior as a method for forming an image at high speed.
However, the insulation liquid used in the conventional liquid developer is mainly composed of a petroleum-based carbon hydride. When such a liquid developer is used, an insulation liquid is adhering to a surface of each toner particle during fixing process of the toner particles.
In the conventional liquid developer, there is a problem in that such an insulation liquid adhering to the surfaces of the toner particles lowers fixing strength of the toner particles to a recording medium and thereby it is impossible to obtain a satisfactory fixing characteristic.
In order to solve such a problem as described above, it is attempted that plant-derived natural oil is used as an insulation liquid and an oxidation polymerization reaction of the oil is utilized for improving fixing strength of toner particles to a recording medium during the fixing process. One example of such a method is disclosed in JP-A 2006-251252.
However, in the liquid developer using the oil as described above, it has been possible to improve fixing strength to a recording medium to a certain level, but the improved fixing strength of the toner particles to a recording medium has not yet reached to a practically required level.